The need for increasing yield and throughput in semiconductor manufacturing systems has resulted in the development of highly specialized and automated systems for processing and handling semiconductor wafers. Wafers are typically stored in a cassette with their flat surfaces horizontal. The cassettes are transferred from station-to-station by automatic material handling equipment. Once delivered to at a process station, individual wafers are transferred from the cassette by an automatically controlled robot that typically includes a robotic arm carrying a wafer supporting blade on its outer end. The blade is moved in a manner to pick up individual wafers such that the wafer lays flat on the blade during the transfer process. This arrangement has the advantage that a wafer is held on the end of the robotic arm through the force of gravity, thus avoiding the application of unnecessary or unbalanced forces to the wafer that could result in damage to the wafer.
Wafer processing systems of the type referred to above typically include one or more so called "cluster tools" consisting of a modular, multi-chambered, integrated processing system having a central wafer handling module, and a number of peripheral process chambers. Cluster tools have become generally accepted as effective and efficient equipment for manufacturing advanced microelectronic devices. Wafers are introduced into a cluster tool where they undergo a series of processing steps sequentially in various circumferentially arranged chambers wherein wafer transfer is effected through the robotic wafer handling module which is located in the central region of the system. The wafer handling module includes a robotic wafer transfer arm which is positioned on a turntable that rotates through 360 degrees, thus permitting the arm to transfer wafers sequentially from chamber-to-chamber.
In spite of the fact that the robotic arm for transferring the wafers is automatically controlled, slight misadjustments, errors caused by wear or software "glitches" can result in problems in the wafer transfer process. For example, in some cases, misalignment of certain components in the system, such as the blade, robotic arm or the cassette can result in a wafer being loaded onto a blade such that the wafer is tilted relative to the blade, i.e. not horizontal on the blade. In other cases, although the wafer may be supported flat on the blade, the blade itself may be tilted after it lifts a wafer from the cassette. In still other cases where the rotational position of the robotic arm relative to a cassette is not precisely indexed or where the blade is tilted out of its normal horizontal position, attempts to fetch a wafer from the cassette may result in the robotic arm or the blade colliding with slit door insert of the load lock chambers which in turn damages or breaks the wafer. Even where the robotic succeeds in lifting a wafer from the cassette, alignment errors in the transfer process can result in collisions between process chamber pedestal, slit door, lower parts of chamber and the wafer during loading, unloading or transfer of the wafers.
The problem described above involving wafer damage due to errors in the transfer process have become more significant as the size of the wafers continues to increase, along with advances in wafer processing technology. This type of wafer damage reduces throughput and process yield, and is becoming more costly due to the larger size of the wafers.
The present invention is directed toward overcoming the deficiencies of the prior art wafer transfer systems described above.